//!
//! It is sometimes useful to have objects that are guaranteed to not move,
//! in the sense that their placement in memory does not change, and can thus be relied upon.
-//!
//! A prime example of such a scenario would be building self-referential structs,
//! since moving an object with pointers to itself will invalidate them,
//! which could cause undefined behavior.
//!
+//! A [`Pin<P>`] ensures that the pointee of any pointer type `P` has a stable location in memory,
+//! meaning it cannot be moved elsewhere and its memory cannot be deallocated
+//! until it gets dropped. We say that the pointee is "pinned".
+//!
//! By default, all types in Rust are movable. Rust allows passing all types by-value,
-//! and common smart-pointer types such as `Box`, `Rc`, and `&mut` allow replacing and
-//! moving the values they contain. In order to prevent objects from moving, they must
-//! be pinned by wrapping a pointer to the data in the [`Pin`] type.
-//! Doing this prohibits moving the value behind the pointer.
-//! For example, `Pin<Box<T>>` functions much like a regular `Box<T>`,
-//! but doesn't allow moving `T`. The pointer value itself (the `Box`) can still be moved,
-//! but the value behind it cannot.
-//!
-//! Since data can be moved out of `&mut` and `Box` with functions such as [`swap`],
-//! changing the location of the underlying data, [`Pin`] prohibits accessing the
-//! underlying pointer type (the `&mut` or `Box`) directly, and provides its own set of
-//! APIs for accessing and using the value. [`Pin`] also guarantees that no other
-//! functions will move the pointed-to value. This allows for the creation of
-//! self-references and other special behaviors that are only possible for unmovable
-//! values.
+//! and common smart-pointer types such as `Box<T>` and `&mut T` allow replacing and
+//! moving the values they contain: you can move out of a `Box<T>`, or you can use [`mem::swap`].
+//! [`Pin<P>`] wraps a pointer type `P`, so `Pin<Box<T>>` functions much like a regular `Box<T>`:
+//! when a `Pin<Box<T>>` gets dropped, so do its contents, and the memory gets deallocated.
+//! Similarily, `Pin<&mut T>` is a lot like `&mut T`. However, [`Pin<P>`] does not let clients
+//! actually obtain a `Box<T>` or `&mut T` to pinned data, which implies that you cannot use
+//! operations such as [`mem::swap`]:
+//! ```
+//! use std::pin::Pin;
+//! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
+//! // `mem::swap` needs `&mut T`, but we cannot get it.
+//! // We are stuck, we cannot swap the contents of these references.
+//! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
+//! // we are not allowed to use it for moving things out of the `Pin`.
+//! }
+//! ```
//!
-//! However, these restrictions are usually not necessary. Many types are always freely
-//! movable. These types implement the [`Unpin`] auto-trait, which nullifies the effect
-//! of [`Pin`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function identically, as do
-//! `Pin<&mut T>` and `&mut T`.
+//! It is worth reiterating that [`Pin<P>`] does *not* change the fact that a Rust compiler
+//! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, `Pin<P>`
+//! prevents certain *values* (pointed to by pointers wrapped in `Pin<P>`) from being
+//! moved by making it impossible to call methods that require `&mut T` on them
+//! (like [`mem::swap`]).
//!
-//! Note that pinning and `Unpin` only affect the pointed-to type. For example, whether
-//! or not `Box<T>` is `Unpin` has no affect on the behavior of `Pin<Box<T>>`. Similarly,
-//! `Pin<Box<T>>` and `Pin<&mut T>` are always `Unpin` themselves, even though the
-//! `T` underneath them isn't, because the pointers in `Pin<Box<_>>` and `Pin<&mut _>`
-//! are always freely movable, even if the data they point to isn't.
+//! [`Pin<P>`] can be used to wrap any pointer type `P`, and as such it interacts with
+//! [`Deref`] and [`DerefMut`]. A `Pin<P>` where `P: Deref` should be considered
+//! as a "`P`-style pointer" to a pinned `P::Target` -- so, a `Pin<Box<T>>` is
+//! an owned pointer to a pinned `T`, and a `Pin<Rc<T>>` is a reference-counted
+//! pointer to a pinned `T`.
+//! For correctness, [`Pin<P>`] relies on the [`Deref`] and [`DerefMut`] implementations
+//! to not move out of their `self` parameter, and to only ever return a pointer
+//! to pinned data when they are called on a pinned pointer.
//!
-//! [`Pin`]: struct.Pin.html
-//! [`Unpin`]: ../../std/marker/trait.Unpin.html
-//! [`swap`]: ../../std/mem/fn.swap.html
-//! [`Box`]: ../../std/boxed/struct.Box.html
+//! # `Unpin`
+//!
+//! However, these restrictions are usually not necessary. Many types are always freely
+//! movable, even when pinned, because they do not rely on having a stable address.
+//! This includes all the basic types (like `bool`, `i32`, references)
+//! as well as types consisting solely of these types.
+//! Types that do not care about pinning implement the [`Unpin`] auto-trait, which
+//! cancels the effect of [`Pin<P>`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function
+//! identically, as do `Pin<&mut T>` and `&mut T`.
+//!
+//! Note that pinning and `Unpin` only affect the pointed-to type `P::Target`, not the pointer
+//! type `P` itself that got wrapped in `Pin<P>`. For example, whether or not `Box<T>` is
+//! `Unpin` has no effect on the behavior of `Pin<Box<T>>` (here, `T` is the
+//! pointed-to type).
//!
-//! # Examples
+//! # Example: self-referential struct
//!
//! ```rust
//! use std::pin::Pin;
//! // let new_unmoved = Unmovable::new("world".to_string());
//! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
//! ```
+//!
+//! # Example: intrusive doubly-linked list
+//!
+//! In an intrusive doubly-linked list, the collection does not actually allocate
+//! the memory for the elements itself. Allocation is controlled by the clients,
+//! and elements can live on a stack frame that lives shorter than the collection does.
+//!
+//! To make this work, every element has pointers to its predecessor and successor in
+//! the list. Elements can only be added when they are pinned, because moving the elements
+//! around would invalidate the pointers. Moreover, the `Drop` implementation of a linked
+//! list element will patch the pointers of its predecessor and successor to remove itself
+//! from the list.
+//!
+//! Crucially, we have to be able to rely on `drop` being called. If an element
+//! could be deallocated or otherwise invalidated without calling `drop`, the pointers into it
+//! from its neighbouring elements would become invalid, which would break the data structure.
+//!
+//! Therefore, pinning also comes with a `drop`-related guarantee.
+//!
+//! # `Drop` guarantee
+//!
+//! The purpose of pinning is to be able to rely on the placement of some data in memory.
+//! To make this work, not just moving the data is restricted; deallocating, repurposing, or
+//! otherwise invalidating the memory used to store the data is restricted, too.
+//! Concretely, for pinned data you have to maintain the invariant
+//! that *its memory will not get invalidated from the moment it gets pinned until
+//! when `drop` is called*. Memory can be invalidated by deallocation, but also by
+//! replacing a [`Some(v)`] by [`None`], or calling [`Vec::set_len`] to "kill" some elements
+//! off of a vector.
+//!
+//! This is exactly the kind of guarantee that the intrusive linked list from the previous
+//! section needs to function correctly.
+//!
+//! Notice that this guarantee does *not* mean that memory does not leak! It is still
+//! completely okay not to ever call `drop` on a pinned element (e.g., you can still
+//! call [`mem::forget`] on a `Pin<Box<T>>`). In the example of the doubly-linked
+//! list, that element would just stay in the list. However you may not free or reuse the storage
+//! *without calling `drop`*.
+//!
+//! # `Drop` implementation
+//!
+//! If your type uses pinning (such as the two examples above), you have to be careful
+//! when implementing `Drop`. The `drop` function takes `&mut self`, but this
+//! is called *even if your type was previously pinned*! It is as if the
+//! compiler automatically called `get_unchecked_mut`.
+//!
+//! This can never cause a problem in safe code because implementing a type that relies on pinning
+//! requires unsafe code, but be aware that deciding to make use of pinning
+//! in your type (for example by implementing some operation on `Pin<&[mut] Self>`)
+//! has consequences for your `Drop` implementation as well: if an element
+//! of your type could have been pinned, you must treat Drop as implicitly taking
+//! `Pin<&mut Self>`.
+//!
+//! In particular, if your type is `#[repr(packed)]`, the compiler will automatically
+//! move fields around to be able to drop them. As a consequence, you cannot use
+//! pinning with a `#[repr(packed)]` type.
+//!
+//! # Projections and Structural Pinning
+//!
+//! One interesting question arises when considering the interaction of pinning and
+//! the fields of a struct. When can a struct have a "pinning projection", i.e.,
+//! an operation with type `fn(Pin<&[mut] Struct>) -> Pin<&[mut] Field>`?
+//! In a similar vein, when can a generic wrapper type (such as `Vec<T>`, `Box<T>`, or `RefCell<T>`)
+//! have an operation with type `fn(Pin<&[mut] Wrapper<T>>) -> Pin<&[mut] T>`?
+//!
+//! Having a pinning projection for some field means that pinning is "structural":
+//! when the wrapper is pinned, the field must be considered pinned, too.
+//! After all, the pinning projection lets us get a `Pin<&[mut] Field>`.
+//!
+//! However, structural pinning comes with a few extra requirements, so not all
+//! wrappers can be structural and hence not all wrappers can offer pinning projections:
+//!
+//! 1. The wrapper must only be [`Unpin`] if all the structural fields are
+//! `Unpin`. This is the default, but `Unpin` is a safe trait, so as the author of
+//! the wrapper it is your responsibility *not* to add something like
+//! `impl<T> Unpin for Wrapper<T>`. (Notice that adding a projection operation
+//! requires unsafe code, so the fact that `Unpin` is a safe trait does not break
+//! the principle that you only have to worry about any of this if you use `unsafe`.)
+//! 2. The destructor of the wrapper must not move structural fields out of its argument. This
+//! is the exact point that was raised in the [previous section][drop-impl]: `drop` takes
+//! `&mut self`, but the wrapper (and hence its fields) might have been pinned before.
+//! You have to guarantee that you do not move a field inside your `Drop` implementation.
+//! In particular, as explained previously, this means that your wrapper type must *not*
+//! be `#[repr(packed)]`.
+//! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
+//! once your wrapper is pinned, the memory that contains the
+//! content is not overwritten or deallocated without calling the content's destructors.
+//! This can be tricky, as witnessed by `VecDeque<T>`: the destructor of `VecDeque<T>` can fail
+//! to call `drop` on all elements if one of the destructors panics. This violates the
+//! `Drop` guarantee, because it can lead to elements being deallocated without
+//! their destructor being called. (`VecDeque` has no pinning projections, so this
+//! does not cause unsoundness.)
+//! 4. You must not offer any other operations that could lead to data being moved out of
+//! the fields when your type is pinned. For example, if the wrapper contains an
+//! `Option<T>` and there is a `take`-like operation with type
+//! `fn(Pin<&mut Wrapper<T>>) -> Option<T>`,
+//! that operation can be used to move a `T` out of a pinned `Wrapper<T>` -- which means
+//! pinning cannot be structural.
+//!
+//! For a more complex example of moving data out of a pinned type, imagine if `RefCell<T>`
+//! had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
+//! Then we could do the following:
+//! ```compile_fail
+//! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>) {
+//! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
+//! let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
+//! let b = rc_shr.borrow_mut();
+//! let content = &mut *b; // And here we have `&mut T` to the same data.
+//! }
+//! ```
+//! This is catastrophic, it means we can first pin the content of the `RefCell<T>`
+//! (using `RefCell::get_pin_mut`) and then move that content using the mutable
+//! reference we got later.
+//!
+//! For a type like `Vec<T>`, both possibilites (structural pinning or not) make sense,
+//! and the choice is up to the author. A `Vec<T>` with structural pinning could
+//! have `get_pin`/`get_pin_mut` projections. However, it could *not* allow calling
+//! `pop` on a pinned `Vec<T>` because that would move the (structurally pinned) contents!
+//! Nor could it allow `push`, which might reallocate and thus also move the contents.
+//! A `Vec<T>` without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents
+//! are never pinned and the `Vec<T>` itself is fine with being moved as well.
+//!
+//! In the standard library, pointer types generally do not have structural pinning,
+//! and thus they do not offer pinning projections. This is why `Box<T>: Unpin` holds for all `T`.
+//! It makes sense to do this for pointer types, because moving the `Box<T>`
+//! does not actually move the `T`: the `Box<T>` can be freely movable (aka `Unpin`) even if the `T`
+//! is not. In fact, even `Pin<Box<T>>` and `Pin<&mut T>` are always `Unpin` themselves,
+//! for the same reason: their contents (the `T`) are pinned, but the pointers themselves
+//! can be moved without moving the pinned data. For both `Box<T>` and `Pin<Box<T>>`,
+//! whether the content is pinned is entirely independent of whether the pointer is
+//! pinned, meaning pinning is *not* structural.
+//!
+//! [`Pin<P>`]: struct.Pin.html
+//! [`Unpin`]: ../../std/marker/trait.Unpin.html
+//! [`Deref`]: ../../std/ops/trait.Deref.html
+//! [`DerefMut`]: ../../std/ops/trait.DerefMut.html
+//! [`mem::swap`]: ../../std/mem/fn.swap.html
+//! [`mem::forget`]: ../../std/mem/fn.forget.html
+//! [`Box<T>`]: ../../std/boxed/struct.Box.html
+//! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len
+//! [`None`]: ../../std/option/enum.Option.html#variant.None
+//! [`Some(v)`]: ../../std/option/enum.Option.html#variant.Some
+//! [drop-impl]: #drop-implementation
+//! [drop-guarantee]: #drop-guarantee
#![stable(feature = "pin", since = "1.33.0")]
where
P::Target: Unpin,
{
- /// Construct a new `Pin` around a pointer to some data of a type that
- /// implements `Unpin`.
+ /// Construct a new `Pin<P>` around a pointer to some data of a type that
+ /// implements [`Unpin`].
+ ///
+ /// Unlike `Pin::new_unchecked`, this method is safe because the pointer
+ /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
+ ///
+ /// [`Unpin`]: ../../std/marker/trait.Unpin.html
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn new(pointer: P) -> Pin<P> {
}
impl<P: Deref> Pin<P> {
- /// Construct a new `Pin` around a reference to some data of a type that
+ /// Construct a new `Pin<P>` around a reference to some data of a type that
/// may or may not implement `Unpin`.
///
+ /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
+ /// instead.
+ ///
/// # Safety
///
/// This constructor is unsafe because we cannot guarantee that the data
- /// pointed to by `pointer` is pinned. If the constructed `Pin<P>` does
- /// not guarantee that the data `P` points to is pinned, constructing a
- /// `Pin<P>` is undefined behavior.
+ /// pointed to by `pointer` is pinned, meaning that the data will not be moved or
+ /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does
+ /// not guarantee that the data `P` points to is pinned, that is a violation of
+ /// the API contract and may lead to undefined behavior in later (safe) operations.
///
- /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
- /// instead.
+ /// By using this method, you are making a promise about the `P::Deref` and
+ /// `P::DerefMut` implementations, if they exist. Most importantly, they
+ /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
+ /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
+ /// and expect these methods to uphold the pinning invariants.
+ /// Moreover, by calling this method you promise that the reference `P`
+ /// dereferences to will not be moved out of again; in particular, it
+ /// must not be possible to obtain a `&mut P::Target` and then
+ /// move out of that reference (using, for example [`mem::swap`]).
+ ///
+ /// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because
+ /// while you are able to pin it for the given lifetime `'a`, you have no control
+ /// over whether it is kept pinned once `'a` ends:
+ /// ```
+ /// use std::mem;
+ /// use std::pin::Pin;
+ ///
+ /// fn move_pinned_ref<T>(mut a: T, mut b: T) {
+ /// unsafe {
+ /// let p: Pin<&mut T> = Pin::new_unchecked(&mut a);
+ /// // This should mean the pointee `a` can never move again.
+ /// }
+ /// mem::swap(&mut a, &mut b);
+ /// // The address of `a` changed to `b`'s stack slot, so `a` got moved even
+ /// // though we have previously pinned it! We have violated the pinning API contract.
+ /// }
+ /// ```
+ /// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`).
+ ///
+ /// Similarily, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be
+ /// aliases to the same data that are not subject to the pinning restrictions:
+ /// ```
+ /// use std::rc::Rc;
+ /// use std::pin::Pin;
+ ///
+ /// fn move_pinned_rc<T>(mut x: Rc<T>) {
+ /// let pinned = unsafe { Pin::new_unchecked(x.clone()) };
+ /// {
+ /// let p: Pin<&T> = pinned.as_ref();
+ /// // This should mean the pointee can never move again.
+ /// }
+ /// drop(pinned);
+ /// let content = Rc::get_mut(&mut x).unwrap();
+ /// // Now, if `x` was the only reference, we have a mutable reference to
+ /// // data that we pinned above, which we could use to move it as we have
+ /// // seen in the previous example. We have violated the pinning API contract.
+ /// }
+ /// ```
+ ///
+ /// [`mem::swap`]: ../../std/mem/fn.swap.html
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
}
/// Gets a pinned shared reference from this pinned pointer.
+ ///
+ /// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`.
+ /// It is safe because, as part of the contract of `Pin::new_unchecked`,
+ /// the pointee cannot move after `Pin<Pointer<T>>` got created.
+ /// "Malicious" implementations of `Pointer::Deref` are likewise
+ /// ruled out by the contract of `Pin::new_unchecked`.
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn as_ref(self: &Pin<P>) -> Pin<&P::Target> {
impl<P: DerefMut> Pin<P> {
/// Gets a pinned mutable reference from this pinned pointer.
+ ///
+ /// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`.
+ /// It is safe because, as part of the contract of `Pin::new_unchecked`,
+ /// the pointee cannot move after `Pin<Pointer<T>>` got created.
+ /// "Malicious" implementations of `Pointer::DerefMut` are likewise
+ /// ruled out by the contract of `Pin::new_unchecked`.
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn as_mut(self: &mut Pin<P>) -> Pin<&mut P::Target> {
unsafe { Pin::new_unchecked(&mut *self.pointer) }
}
- /// Assign a new value to the memory behind the pinned reference.
+ /// Assigns a new value to the memory behind the pinned reference.
+ ///
+ /// This overwrites pinned data, but that is okay: its destructor gets
+ /// run before being overwritten, so no pinning guarantee is violated.
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn set(self: &mut Pin<P>, value: P::Target)
}
impl<'a, T: ?Sized> Pin<&'a T> {
- /// Construct a new pin by mapping the interior value.
+ /// Constructs a new pin by mapping the interior value.
///
/// For example, if you wanted to get a `Pin` of a field of something,
/// you could use this to get access to that field in one line of code.
+ /// However, there are several gotchas with these "pinning projections";
+ /// see the [`pin` module] documentation for further details on that topic.
///
/// # Safety
///
/// will not move so long as the argument value does not move (for example,
/// because it is one of the fields of that value), and also that you do
/// not move out of the argument you receive to the interior function.
+ ///
+ /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
#[stable(feature = "pin", since = "1.33.0")]
pub unsafe fn map_unchecked<U, F>(self: Pin<&'a T>, func: F) -> Pin<&'a U> where
F: FnOnce(&T) -> &U,
/// Gets a shared reference out of a pin.
///
+ /// This is safe because it is not possible to move out of a shared reference.
+ /// It may seem like there is an issue here with interior mutability: in fact,
+ /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
+ /// not a problem as long as there does not also exist a `Pin<&T>` pointing
+ /// to the same data, and `RefCell<T>` does not let you create a pinned reference
+ /// to its contents. See the discussion on ["pinning projections"] for further
+ /// details.
+ ///
/// Note: `Pin` also implements `Deref` to the target, which can be used
/// to access the inner value. However, `Deref` only provides a reference
/// that lives for as long as the borrow of the `Pin`, not the lifetime of
/// the `Pin` itself. This method allows turning the `Pin` into a reference
/// with the same lifetime as the original `Pin`.
+ ///
+ /// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn get_ref(self: Pin<&'a T>) -> &'a T {
///
/// For example, if you wanted to get a `Pin` of a field of something,
/// you could use this to get access to that field in one line of code.
+ /// However, there are several gotchas with these "pinning projections";
+ /// see the [`pin` module] documentation for further details on that topic.
///
/// # Safety
///
/// will not move so long as the argument value does not move (for example,
/// because it is one of the fields of that value), and also that you do
/// not move out of the argument you receive to the interior function.
+ ///
+ /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
#[stable(feature = "pin", since = "1.33.0")]
pub unsafe fn map_unchecked_mut<U, F>(self: Pin<&'a mut T>, func: F) -> Pin<&'a mut U> where
F: FnOnce(&mut T) -> &mut U,
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